Magnetic tape is mounted on tape reels with one or more read/write heads of a tape drive positioned adjacent a length of the magnetic tape. The magnetic tape is either provided with one tape reel and threaded to a second tape reel in the tape drive, or is mounted on two tape reels in a cartridge. The magnetic tape is moved longitudinally, typically at a high speed, between the tape reels, to access desired data on the magnetic tape, and then moved at a lower speed to read and/or write data with respect to the magnetic tape.
Lowering the time to access data on a magnetic tape is typically considered important, and is often countered by having more data on the magnetic tape by making the magnetic tape longer. One solution is to move the magnetic tape longitudinally at ever higher velocities. In magnetic tape drives, the tape is typically guided in the lateral direction to provide an alignment with respect to the read/write head(s). Tape path guides provide such lateral guiding.
One type of tape path guide is a fixed guide, which may comprise ceramic or metal guides on either side of the tape path. At high tape velocities, the magnetic tape tends to abrade and cut into the guides, reducing their effective life, and may distort or otherwise damage the magnetic tape. The fixed guide typically also comprises a fixed surface over which the tape slides, which generates friction with the tape and tends to distort and stretch the tape, and, as such, is not usable for high velocities. Rather, such tape guides employ air bearings to allow the tape to ride on a cushion of air. However, air bearings require air pressure pumps and plumbing, resulting in inefficient use of space and resulting in extra cost.
Another type of tape path guide is a tape path roller guide which is a roller with flanges on either side of the tape path. Smooth tape path roller guides tend to float the magnetic tape, causing it to slide. Hence, many tape path roller guides are provided with grooves that extend circumferentially with respect to the roller, or have a circumferential aspect, so that air that would otherwise be trapped under the magnetic tape as the magnetic tape is brought into contact with the roller, is allowed to move into the groove and moves circumferentially along the groove away from the point of contact, and is bled off. However, at high tape velocities and high roller velocities, the friction between the air and the magnetic tape, and between the air and the roller surface, is too great, such that the air becomes trapped and the magnetic tape is partially or entirely floated off the roller, causing the magnetic tape to slide on the surface of the roller. This leads to abrasion of both the magnetic tape and the roller, and to distortion of the magnetic tape. Increasing the tension on the magnetic tape to counteract the effect may cause the tape to stretch and distort.
Additionally, if the grooves are entirely circumferential, the edges between the grooves and surface of the roller tend to mishape the tape and to cut into and distort the magnetic tape. If the grooves are partially circumferential, such as helical or crosshatched, the grooves tend to move the tape from one side of the roller to the other, dependent on the variations in tension on the magnetic tape.
Further, the grooves must be machined. If the grooves are helical or crosshatched, they cannot be molded, since the release of the mold would cause undercuts to the roller. If the grooves are entirely circumferential, the witness line, or mold separation line, would have to be axial to prevent undercuts, resulting in the need to machine the entire surface to insure roundness. Hence, the grooves of roller guides are typically machined.